This invention relates generally to electrophotographic printing, and more particularly concerns an optical system employed therein for forming a half-tone light image of an original document being reproduced.
Screening achieves the effect of tone graduation by means of dot size variations. In the highlight regions, the dots are small and increase in size through the intermediate shades until they merge together in the shadow regions. At the highlight end of the tone scale, there is complete whiteness, while at the shadow end, nearly solid blackness. This type of tone structure can be reproduced in an electrophotographic printing machine.
Many techniques have been developed to improve half-tone reproduction. Moire patterns are minimized by re-orienting the screen between successive single color half-tone patterns.
With the advent of color electrophotographic printing, screening techniques have been employed to improve copy quality. Multi-color electrophotographic printing is similar to black and white printing. The process of black and white electrophotographic printing is described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942. In color electrophotographic printing, the light image is filtered producing successive single color light images of the original document. These colored light images expose a charged photoconductive surface to create successive single color electrostatic latent images thereon. Each single color electrostatic latent image is developed with toner particles complementary in color to the color of the filtered light image. The toner powder images are transferred from the electrostatic latent image to a sheet of support material, in superimposed registration with one another. This multi-colored powder image is permanently affixed to the sheet of support material forming a color copy thereon.
Half-tone screens are used in multi-color electrophotographic printing systems to enhance the copy being reproduced. The screen is interposed into the optical light path and successive single color light images are transmitted therethrough onto the charged photoconductive surface to record an image of dots. By proper angular positioning of the half-tone screen, moire patterns can be avoided. A minimum pattern is formed when a screen angle of 30.degree. is used between successive images. Since screens comprise line rulings or dots along axis of 90.degree. relative to one another, only three angles, separated by 30.degree., may be employed before the angles repeat. In four-color printing, two of the colors must be separated by an angle other than 30.degree.. Generally, inasmuch as yellow is a light color, it is printed at a 15.degree. angular separation between two other colors, generally cyan and magenta. Usually the screen angles are: black, 45.degree.; magenta, 75.degree.; yellow, 90.degree.; and cyan, 105.degree.. An error as small as 0.1.degree. between screen angles or a slight misregistration between colors can cause moire patterns in areas where three or four colors print. U.S. Pat. No. 3,109,239 issued to Wicker et al. in 1963 discloses the use of half-tone screens wherein the screen angle is separated by 30.degree.for red, blue and black color images. The yellow image is interposed between the black and blue images. Similarly, U.S. Pat. No. 3,381,612 issued to Lecha in 1968 teaches the concept of separating the cyan, magenta and black images by 30.degree.while interposing the yellow image between the magenta and black images. However, none of the prior art references appear to solve the problem of extremely tight angular tolerances being required between screen angles, i.e., such as 0.1.degree.. It has been found in three color printing that the angular tolerance between screen angles is not as critical as is the case for four color printing.
Accordingly, it is a primary object of the present invention to improve electrophotographic printing machines by reducing the angular tolerance requirement between successive different screen angles while minimizing moire patterns.